1. Field of the Invention
The present invention relates generally to USB devices, and more specifically to overcoming the limited common-mode range for USB systems.
2. Description of the Related Art
The Universal Serial Bus (USB) was developed to offer PC users an enhanced and easy-to-use interface for connecting an incredibly diverse range of peripherals to their computers. The development of the USB was initially driven by considerations for laptop computers, which greatly benefit from a small profile peripheral connector. Among the many benefits of the USB is a reduction in the proliferation of cables that can affect even the smallest computer installations. In general, USB has become the interface of choice for PCs because it offers users simple connectivity. USB eliminates the need to have different connectors for printers, keyboards, mice, and other peripherals, and supports a wide variety of data types, from slow mouse inputs to digitized audio and compressed video. In addition, USB devices are hot pluggable, i.e. they can be connected to or disconnected from a PC without requiring the PC to be powered off.
The USB specification has seen various revisions, with the USB 2.0 standard challenging the IEEE 1394 interface (“Firewire”) as the interface of choice for high-speed digital video, among others. The USB 3.0 standard, representing the second major revision of the USB standard, specifies a maximum transmission speed of up to 5 Gbits/s (640 MBbytes/s), which is over 10 times faster than the maximum speed specified in the USB 2.0 standard (480 Mbits/s). The USB 3.0 standard also features reduced time required for data transmission, reduced power consumption, and is backward compatible with USB 2.0. With the proliferating design of increasingly smarter, faster, and smaller peripherals, the On-The-Go (OTG) Supplement to the USB 2.0 Specification was also developed to address the growing popularity of the portable electronic devices market. OTG devices typically do not require a PC host, and can communicate directly with each other. For example, a PDA may act as a USB host with the capability to print directly to a USB printer, while also acting as a USB peripheral to communicate with a PC. In general, designers are facing increasing pressure to design smaller and faster products in less time and at lower cost.
Present day USB devices that communicate with a host over USB include USB printers, scanners, digital cameras, storage devices, card readers, etc. USB based systems may require that a USB host controller be present in the host system, and that the operating system (OS) of the host system support USB and USB Mass Storage Class Devices. USB devices may communicate over the USB bus at low-speed (LS), full-speed (FS), or high-speed (HS). A connection between the USB device and the host may be established via a four-wire interface that includes a power line, a ground line, and a pair of data lines D+ and D−. When a USB device connects to the host, the USB device may first pull a D+ line high (the D− line if the device is a low speed device) using a pull up resistor on the D+ line. The host may respond by resetting the USB device. If the USB device is a high-speed USB device, the USB device may “chirp” by driving the D− line high during the reset. The host may respond to the “chirp” by alternately driving the D+ and D− lines high. The USB device may then electronically remove the pull up resistor and continue communicating at high speed. When disconnecting, full-speed devices may remove the pull up resistor from the D+ line (i.e., “tri-state” the line), while high-speed USB devices may tri-state both the D+ and D− lines.
The USB standard provides very stringent guidelines for the allowed common-mode voltage on the differential data lines (D+ and D−). On the other hand, newer specifications allow for battery charging using a USB port, which oftentimes results in currents that are much higher than initially specified for USB, producing higher voltage drops across the USB setups. This can lead to the need for thicker cables and/or reduced cable length when trying to communicate across the USB while the attached USB device is charging. Such change in cabling needs may not allow full backward compatibility for existing cabling infrastructures.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.